The present invention relates to liquid crystal displays formed on silicon-on-sapphire. More specifically, but without limitation thereto, the present invention relates to a liquid crystal display integrated with electronic circuitry on the display to provide a programmable gray-scale and to compensate for nonuniform and non-operating pixels in the display.
Liquid crystal displays (LCDs) are used in a wide variety of commercial applications, including portable and laptop computers, wristwatches, camcorders, and television screens. Inherent limitations of existing technology arise from the necessity of fabricating LCDs on transparent glass or quartz substrates which are not amenable to processing with high quality electronic materials.
The integration of drive circuitry with LCDs has improved reliability and reduced size and weight for portable applications, but has been limited to thin film transistor technology using, for example, amorphous (α-Si) and polycrystalline (poly-Si) silicon deposited on glass and quartz substrates.
Lattice and thermal mismatch between layers and low temperature deposition methods used in thin film transistor technology result in a silicon layer with poor charge carrier mobility and crystallographic defects which are directly related to electronic device performance and limitations. A comparison of MOS technologies for active matrix LCDs is shown in the following table:
POLY-TFTPOLY-TFTα-Si:HCMOSHT-CMOSMT-CMOSNMOSUTSOS1. Substratefused quartzhard glasshard glassAl2O32. Max.~1000° C.600° C.300° C.1000° C.process temp3. Threshold2.02.01.50.5(Volts)(n-chnl)4. Mobility100400.753805. Shift20 MHz5 MHz0.1 MHz>100 MHzregister@15 V@15 V@15 V@5 V6. IntegratedN/AN/AN/AyesLSI logic
For ultra-high resolution display applications, the high density of LSI circuitry is of particular importance for integrated displays. Compatibility with Very Large Scale Integration (VLSI) allows integration on-chip of video drivers, digital logic, compensating or fault-tolerant circuitry, and other computational circuitry, thereby providing greater functionality, higher reliability, and improved performance. A need thus exists for a material quality that overcomes the problems which occur in small scale, high density circuitry fabricated in α-Si and poly-Si.
A need also exists for multiple level gray-scale and color displays for the applications mentioned above. Color displays have been made with colored filters by incorporating dyes into a guest host matrix, or by using field sequential color techniques. Color liquid crystal displays may also be made using the gray-scale properties of a liquid crystal display to achieve variations in color.
While the optical, electrical, and electro-optical properties of the liquid crystal material primarily determine the gray-scale properties, the substrate plays a significant role in the pixel uniformity of the display. Substrate warpage, or variations in surface morphology, can lead to variations in thickness of the liquid crystal layer. This in turn may lead to a nonuniform display intensity for a given pixel voltage, which is a problem for multiple gray-scale displays, high density displays, and displays having stringent operating requirements. Furthermore, for high brightness displays, substantial heating may occur which can not be readily dissipated through substrates such as glass or quartz.
Prior research on brightness nonuniformity of LCDs established another cause of display nonuniformity, specifically the high resistance of narrow electrodes in high density LCDs.
A related problem particularly important for displays having stringent specifications is fault tolerance, or recovering from failed pixels. This problem is not emphasized in an LCD market primarily interested in low cost commercial applications, but becomes significant in high-reliability technology.
Another problem is that as display resolutions increase, the number of switching elements required in active matrix displays increases. A higher number of switching elements causes yield problems in manufacturing and in reliability. Fabrication yields of nonlinear switching elements (thin film transistors or diodes) may be improved by redundancy, but the redundancy applies only to the switching element rather than for the entire pixel.